In recent years, in the course of combating global warming, there is a demand for a high-capacity power generating plant, and high-efficiency ultra super critical power generation boilers have been developed actively. Also, with the prevalence of oil depletion problem, an oil and natural gas exploitation environment has become much more hostile. In the power generation boilers, oil wells, and gas wells, used is a seamless tube which is excellent in strength, corrosion resistance, and stress corrosion cracking resistance, and the material grade of the seamless tube tends to be high-Cr and high-Ni alloys in response to such escalated requirements in application.
Because of poor workability of high-Cr and high-Ni materials, there are growing demands for seamless tubes produced by a hot extrusion tube-making process, as a method for producing tubes from such hard-to-work materials in which features in high working speed, less temperature drop of in-process material, and achieving a high reduction rate. In particular, the Ugine-Sejournet process characterized by glass lubrication is suitable for producing a seamless tube from a hard-to-work material.
FIG. 1 is a sectional view for illustrating the hot extrusion tube-making process for making a seamless tube by using the Ugine-Sejournet process. As shown in FIG. 1, in the Ugine-Sejournet process, a hollow starting material to be extruded (hereinafter, also referred to as a “billet”) 8 with a through hole formed in along the axial centerline thereof is heated, and the billet 8 heated to a predetermined temperature is housed in a container 6. Thereafter, with a mandrel bar 3 inserted in the axial center of the billet 8, the billet 8 is extruded via a dummy block 7 by the movement (in the direction indicated by the hollow arrow in FIG. 1) of a stem along with a ram, not shown, being driven to produce an extruded tube as being a seamless tube.
At this time, a die 2 held by a die holder 4 and a die backer 5 is arranged at the front end of the container 6, and the billet 8 is extruded in the stem movement direction through an annular gap formed by the inner surface of the die 2 and the outer surface of the mandrel bar 3 to form an extruded tube having a desired outside diameter and wall thickness.
In the Ugine-Sejournet process, glass is used as a lubricant. Before the billet 8 is housed in the container 6, powder glass is provided onto the outer surface and the inner surface of the heated billet 8 to form a film of molten glass. This glass film lubricates between the billet 8 and the container 6 as well as between the billet 8 and the mandrel bar 3.
In addition, a glass disc 1 formed in an annular shape by mixing powder glass with glass fiber and water glass is mounted between the billet 8 and the die 2. This glass disc 1 is melted gradually in the process of extrusion by the heat retained by the billet 8, and lubricates between the billet 8 and the die 2.
In the above-described hot extrusion tube-making process, the billet temperature during extrusion depends on the billet heating temperature, the heat dissipation caused by heat transfer to tools (container, mandrel bar, and die), and the heat generation associated with plastic deformation. If the heat dissipation of billet is significant, the billet temperature decreases, and the deformation resistance increases, so that the load imposed on the tube-making equipment becomes excessive, which may result in incompletion of extrusion and hence may become a hindrance in terms of operation and yield. If the billet heating temperature is increased excessively to avoid the problem, flaws occur on the extruded tube because of decreasing into a low ductility region in the high-temperature zone, and the yield is decreased by the product defective. In particular, on the outer surface of the top portion (the portion of the extrusion front) of extruded tube, flaws in a transverse direction, which is called a transverse/lateral flaw, is prone to occur.
In general, the high-Cr and high-Ni materials have high deformation resistance, and temperatures exhibiting good high-temperature ductility (the temperature at which the reduction of area is 90% or more in the high-temperature tensile test) are low, and the range of the temperatures is narrow, so that the deformability is low at high temperatures. Therefore, in the hot extrusion using a high-Cr and high-Ni materials as starting material to be extruded, the hindrance in terms of operation and yield caused by the incompletion of extrusion and the decrease in yield caused by flaws on the extruded tube become significant. Therefore, in order to produce a high-quality extruded tube by using a billet having low deformability at high temperatures, it is necessary to grasp the ductility decreasing temperature in the high-temperature zone and also to take into consideration the processing-incurred heat.
As a method for ensure the quality of extruded tube, for example, Patent Literatures 1 and 2 disclose a method for extruding a metal material, in which a conditional expression based on the container temperature is defined, and extrusion is performed so that the temperature of extruded tube remains constant.